Created attachment 195940 [details]
proposed patch

GtkNotebook's paint method assumes that if the current page is non-NULL, its allocation has been populated.  The allocation is populated in a size-allocate idle callback, while painting is called in response to the draw signal, which may be emitted in another idle callback, with a higher priority.  If the paint method is called when the page is non-NULL but the allocation is still initialized to zeros, it calls gtk_render_frame_gap with a negative offset, which then emits a critical warning.

The paint method would be more robust if it checked for a populated page allocation--patch attached.